The Core iOS Developer’s Cookbook, Fifth Edition

Monitoring the Battery State

You can programmatically keep track of the battery and charging state. APIs enable you to know the level to which the battery is charged and whether the device is plugged into a charging source. The battery level is a floating-point value that ranges between 1.0 (fully charged) and 0.0 (fully discharged). It provides an approximate discharge level that you can use to query before performing operations that put unusual strain on the device.

For example, you might want to caution your user about performing a large series of mathematical computations and suggest that the user plug in to a power source. You retrieve the battery level via this UIDevice call, and the value returned is produced in 5% increments:

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